$$\require{cancel}$$
The field at a distance $$r$$ from a large charged sheet carrying a charge $$σ$$ coulombs per square metre is  $$\frac{\sigma}{2\epsilon_0}$$. Therefore the potential difference between two points at distances $$a \text{ and }b$$ from the sheet $$(a < b)$$ is
$V_a-V_b = \frac{\sigma}{2\epsilon_0}(b-a).\tag{2.2.7}$